THE CLYDE SEA-AREA. 397 



Influence of Wind. We have so far endeavoured to deduce from the 

 observations actually made some approximation to the normal condition of 

 tilings which would exist if the water were never subject to very strong 

 winds, but it is necessary to refer to the great irregularities which a long 

 continued strong wind produces. These effects are most strikingly shown in 

 the narrow mountain-girdled sea-lochs, where the wind usually blows either 

 up or down the valley. In the case of a strong continuous up-loch wind the 

 Hood-tide is accelerated and the ebb retarded ; but the fresher surface- water^ 

 which in summer is warmer and in winter colder than that beneath, is 

 accumulated by a sort of skimming at the head of the loch, thus tending to 

 raise the level and so forcing away the deeper salt layers. At the same time 

 the reaction currents set up by the stream of water entering the loch cause 

 an up welling of the salter deep water on the bar at the mouth both from 

 outside and inside ; and if the wind were to last long enough the whole mass 

 of water in the loch-basin would be thoroughly mixed and reduced through- 

 out to a uniform temperature and a uniform salinity. It may be that the 

 rapid fall of temperature to the spring minimum is partly due to the 

 frequency of heavy gales in winter. 



When a down-loch wind is blowing the conditions are reversed. Then the 

 ebb-tide is accelerated and the flood retarded on the surface. At the same 

 time the fresher surface-water is driven down, and perhaps out of the loch, 

 and its place taken by the salter water drawn up from below. This accounts 

 for the paradox that the saltest surface water in the whole Clyde Sea-area 

 has been found at the head of the lochs where the largest rivers flow in,, 

 during a down-loch wind ; while the freshest bottom water has been found 

 in the same positions with an up-loch wind. Cross-loch winds produce 

 similar effects, but these occur less frequently, and except when specially 

 investigated have usually escaped attention, as the regular observations, 

 designed to investigate the seasonal changes of temperature and salinity 

 were usuallv made in mid-channel where the effect is at a minimum. 



[Bibliography. 



